SYNCHRONOUS GATING OF BILATERAL SWITCHES IN A THREE PHASE SYSTEM

摘要

1,227,963. Balancing three-phase systems; control of A.C. motors. GENERAL ELECTRIC CO. 13 June, 1968 [7 July, 1967], No. 28264/68. Headings H2H and H2J. A balanced three-phase system includes a pair of back-to-back thyristors or a triac in each of two of the phase lines. The gating of the thyristors is controlled by a synchronizing circuit so that one line is gated in advance of its phase and the other line is gated after its phase. The system may be used to control an induction motor. In the circuit shown, the firing angles of the thyristors 15, 17, 23, 25 are adjusted by a control input 107 feeding trigger circuits 97, 99. Synchronizing signals are fed to the trigger circuits from a three-phase transformer 95, and further transformers 91, 93, whose primaries are connected across phase lines 2 and 3 and phase lines 1 and 2 respectively, adjust the timing of the gate signals from the trigger circuits 97, 99. In the example shown, transformer 93 advances the signal from circuit 99 by 30 degrees, and transformer 91 delays the signal from circuit 97 by the same amount. The system is particularly applicable to controlling the speed of induction motors in that it eliminates single phase currents at large retard angles, and also moves the point of maximum unbalance to small retard angles where it is least important.